Radiofrequency focusing of magnetic resonance images

Abstract

Apparati and methods for magnetic resonance imaging a selected interrogation volume in a tissue of a human or animal body, to provide increased signal-to-noise ratios for fixed data acquisition times. The method involves excitation of magnetic resonance in a selected interrogation volume that may be as small as 500-3,000 cm 3 , through controllable focusing or steering of a rotating magnetic field signal used to induce magnetic resonance. The response signals issued by the excited volume element are then collected by focusing of these response signals, using a phased array of antennae for this purpose. Use of the invention with well known nuclear magnetic resonance excitation procedures, such as spin echo, echo planar, gradient recalled and backprojection, are discussed.

I claim: 
     
       1. Apparatus for producing a time dependent magnetic field in a human or animal body for purposes of magnetic resonance imaging of tissue within an interrogation volume of reduced size no larger than 3,000 cm 3  in the body, and for sensing response signals issued by the tissue in response to receipt by the tissue of magnetic resonance excitation signals, the apparatus comprising: a phased array of antennae, having at least first and second antennae, positioned outside and adjacent to the body in a first coronal plane, that produces a radiofrequency magnetic field B 1  with a field vector that rotates in a second coronal plane, approximately parallel to the first coronal plane, within the body with approximately constant angular frequency, the phased array being positioned (1) to selectively focus the magnetic field B 1  within a selected interrogation volume of size no larger than 3,000 cm 3  within the body and (2) to selectively and preferentially receive response signals from the selected interrogation volume that are issued in response to magnetic resonance excitation of nuclei in the selected interrogation volume;   a first coil or other structure for producing a primary magnetic field B 0  of approximately constant amplitude whose field vector is perpendicular to the second coronal plane;   a second coil or other structure for producing a gradient magnetic field B 2 , parallel to the primary magnetic field whose amplitude is no larger than the amplitude of the magnetic field B 1 , where the gradient field distinguishes between changes of position within the body in at least two mutually orthogonal directions;   signal processing means for accepting the response signals received at each of the antennae as a function of time and for introducing a predetermined phase shift into each such response signal relative to the phases of response signals received at each of the other antennae;   a first switched power supply, connected to the second coil or magnetic field structure, to selectively activate and deactivate the gradient magnetic field at predetermined times; and   a second switched power supply to selectively activate and deactivate the signal processing means at predetermined times.   
     
     
       2. The apparatus of claim 1, further comprising an impedance-matching material positioned to fill substantially all the volume between said phased array of antennae and said body and to approximately match the electrical impedance of said body. 
     
     
       3. The apparatus of claim 1, wherein said primary magnetic field B 0  has a magnitude that lies in the range 2-10 Tesla. 
     
     
       4. The apparatus of claim 1, wherein said signal processing means comprises: first and second amplifiers, positioned to receive first and second response signals, respectively, that are received at a first antenna and at a second antenna, respectively, in said phased array of said antennae, and to issue as output signals amplified first response signals and amplified second response signals, respectively;   first and second local oscillators for producing and issuing first and second local oscillator signals of predetermined frequencies;   first and second heterodyne receivers for receiving the amplified first and second response signals from the first and second amplifiers, respectively, for receiving the first and second local oscillator signals, for forming and issuing a first product signal that is a product of the amplified first response signal and the first local oscillator signal, and for forming and issuing a second product signal that is a product of the amplified second response signal and the second local oscillator signal;   first and second phase shift means for receiving the first product signal, for introducing predetermined first and second phase shifts, respectively, in the first product signal, and for issuing these phase shifted signals as first and second phase shift means output signals;   third and fourth phase shift means for receiving the second product signal, for introducing predetermined third and fourth phase shifts, respectively, in the second product signal, and for issuing these phase shifted signals as third and fourth phase shift means output signals;   a first signal summing device, positioned to receive and form the sum of the first and third phase shift means output signals and to issue this sum signal as a first summing device output signal;   a second signal summing device, positioned to receive and form the sum of the second and fourth phase shift means output signals and to issue this sum signal as a second summing device output signal;   first and second analog-to-digital converters, positioned to receive the first and second summing device output signals, respectively, to convert these signals to digital signals, and to issue these converted digital signals as first and second converter output signals;   signal storage means for receiving and storing the first and second converter output signals;   transform means for forming and issuing a two-dimensional Fourier transform of the first converter output signal and for forming and issuing a two-dimensional Fourier transform signal of the second converter output signal; and   display means for receiving and graphically displaying the Fourier transform signals issued by the transform means.   
     
     
       5. The apparatus of claim 1, wherein said signal processing means comprises: first and second amplifiers, positioned to receive first and second response signals, respectively, that are received at a first antenna and a second antenna, respectively, in said phased array of said antennae, and to issue as output signals amplified first and second response signals;   first and second local oscillators for producing and issuing first and second local oscillator signals of predetermined frequencies;   first and second heterodyne receivers for receiving the amplified first and second amplified response signals from the first and second amplifiers, respectively, for receiving the first and second local oscillator signals, for forming and issuing a first product signal that is a product of the amplified first response signal and the first local oscillator signal, and for forming and issuing a second product signal that is a product of the amplified second response signal and the second local oscillator signal;   first and second analog-to-digital converters, positioned to receive the first and second product signals, respectively, to convert these signals to first and second digital signals, and to issue these converted signals as converted first and second product signals;   first and second memory means for receiving and storing the converted first and second product signals;   first and second phase shift means for receiving the converted first and second product signals from the first and second memory means, for introducing predetermined first and second phase shifts, respectively, in the converted first and second product signals, for issuing these phase shifted signals as first and second phase shift means output signals at a first selected time, for introducing third and fourth predetermined phase shifts, respectively, in the converted first and second product signals, and for issuing these phase shifted signals as third and fourth phase shift means output signals at a second selected time;   a signal summing device for receiving and forming the sum of the first and second phase shift means output signals at the first selected time, for issuing this sum as a first sum signal, for receiving and forming the sum of the third and fourth phase shift means output signals at the second selected time, and for issuing this sum as a second sum signal;   transform means for receiving the first and second sum signals, for forming and issuing a two-dimensional Fourier transform of the first sum signal as a first output signal, and for forming and issuing a two-dimensional Fourier transform of the second sum signal as a second output signal; and   display means for receiving and graphically displaying the first and second output signals issued by the transform means.   
     
     
       6. The apparatus of claim 1, further comprising steering means connected to said sensing antenna array for controllably causing said antenna array to illuminate a selected interrogation volume at a selected position within said tissue. 
     
     
       7. Apparatus for producing a time dependent magnetic field in a human or animal body for purposes of magnetic resonance imaging of tissue within an interrogation volume of reduced size no larger than 3,000 cm 3  in the body, and for sensing response signals issued by the tissue in response to receipt by the tissue of magnetic resonance excitation signals, the apparatus comprising: a phased array of antennae, having at least first and second antennae, positioned outside and adjacent to the body in a first sagittal plane, that produces a radiofrequency magnetic field B 1  with a field vector that rotates in a coronal plane within the body with approximately constant angular frequency, the phased array being positioned (1) to selectively focus the magnetic field B 1  within a selected interrogation volume of size no larger than 3,000 cm 3  within the body and (2) to selectively and preferentially receive response signals from the selected interrogation volume that are issued in response to magnetic resonance excitation of selected nuclei in the selected interrogation volume;   a first coil or other magnetic field structure for producing a primary magnetic field B 0  of approximately constant amplitude whose field vector is perpendicular to the coronal plane;   a second coil or other structure for producing a gradient magnetic field B 2 , parallel to the primary magnetic field whose amplitude is no larger than the amplitude of the magnetic field B 1 , where the gradient field distinguishes between changes of position within the body in at least two mutually orthogonal directions;   signal processing means for accepting the response signals received at each of the antennae as a function of time and for introducing a predetermined phase shift into each such response signal relative to the phases of response signals received at each of the other antennae;   a first switched power supply, connected to the second coil or magnetic field structure, to selectively activate and deactivate the gradient magnetic field at predetermined times; and   a second switched power supply to selectively activate and deactivate the signal processing means at predetermined times.   
     
     
       8. The apparatus of claim 7, further comprising an impedance-matching material positioned to fill substantially all the volume between said phased array of antennae and said body and to approximately match the electrical impedance of said body. 
     
     
       9. The apparatus of claim 7, wherein said primary magnetic field B 0  has a magnitude that lies in the range 2-10 Tesla. 
     
     
       10. The apparatus of claim 7, wherein said signal processing means comprises: first and second amplifiers, positioned to receive first and second response signals, respectively, that are received at a first antenna and a second antenna, respectively, in said phased array of said antennae, and to issue as output signals amplified first response signals and amplified second response signals, respectively;   first and second local oscillators for producing and issuing first and second local oscillator signals of predetermined frequencies;   first and second heterodyne receivers for receiving the amplified first and second response signals from the first and second amplifiers, respectively, for receiving the first and second local oscillator signals, for forming and issuing a first product signal that is a product of the amplified first response signal and the first local oscillator signal, and for forming and issuing a second product signal that is a product of the amplified second response signal and the second local oscillator signal;   first and second phase shift means for receiving the first product signal, for introducing predetermined first and second phase shifts, respectively, in the first product signal, and for issuing these phase shifted signals as first and second phase shift means output signals;   third and fourth phase shift means for receiving the second product signal, for introducing predetermined third and fourth phase shifts, respectively, in the second product signal, and for issuing these phase shifted signals as third and fourth phase shift means output signals;   a first signal summing device, positioned to receive and form the sum of the first and third phase shift means output signals and to issue this sum signal as a first summing device output signal;   a second signal summing device, positioned to receive and form the sum of the second and fourth phase shift means output signals and to issue this sum signal as a second summing device output signal;   first and second analog-to-digital converters positioned to receive the first and second summing device output signals, respectively, to convert these signals to digital signals, and to issue these converted digital signals as first and second converter output signals;   signal storage means for receiving and storing the first and second converter output signals;   transform means for forming and issuing a two-dimensional Fourier transform of the first converter output signal and for forming and issuing a two-dimensional Fourier transform signal of the second converter output signal; and   display means for receiving and graphically displaying the Fourier transform signals issued by the transform means.   
     
     
       11. The apparatus of claim 7, wherein said signal processing means comprises: first and second amplifiers, positioned to receive first and second response signals, respectively, that are received at a first antenna and a second antenna, respectively, in said phased array of said antennae, and to issue as output signals amplified first and second response signals;   first and second local oscillators for producing and issuing first and second local oscillator signals of predetermined frequencies;   first and second heterodyne receivers for receiving the amplified first and second response signals from the first and second amplifiers, respectively, for receiving the first and second local oscillator signals, for forming and issuing a first product signal that is a product of the amplified first response signal and the first local oscillator signal, and for forming and issuing a second product signal that is a product of the amplified second response signal and the second local oscillator signal;   first and second analog-to-digital converters, positioned to receive the first and second product signals, respectively, to convert these signals to digital signals, and to issue these converted signals as converted first and second product signals;   first and second memory means for receiving and storing the converted first and second product signals;   first and second phase shift means for receiving the converted first and second product signals from the first and second memory means, for introducing predetermined first and second phase shifts, respectively, in the converted first and second product signals, for issuing these phase shifted signals as first and second phase shift means output signals at a first selected time, for introducing third and fourth predetermined phase shifts, respectively, in the converted first and second product signals, and for issuing these phase shifted signals as third and fourth phase shift means output signals at a second selected time;   a signal summing device for receiving and forming the sum of the first and second phase shift means output signals at the first selected time, for issuing this sum as a first sum signal, for receiving and forming the sum of the third and fourth phase shift means output signals at the second selected time, and for issuing this sum as a second sum signal;   transform means for receiving the first and second sum signals, for forming and issuing a two-dimensional Fourier transform of the first sum signal as a first output signal, and for forming and issuing a two-dimensional Fourier transform of the second sum signal as a second output signal; and   display means for receiving and graphically displaying the first and second output signals issued by the transform means.   
     
     
       12. The apparatus of claim 7, further comprising steering means connected to said sensing antenna array for controllably causing said antenna array to illuminate a selected interrogation volume at a selected position within said tissue. 
     
     
       13. Apparatus for producing a time dependent magnetic field in a human or animal body for purposes of magnetic resonance imaging of tissue within an interrogation volume of reduced size no larger than 3,000 cm 3  in the body, and for sensing response signals issued by the tissue in response to receipt by the tissue of magnetic resonance excitation signals, the apparatus comprising: a phased array of antennae, having at least first and second antennae, positioned around the body that produces a radiofrequency magnetic field B 1  with a field vector that rotates in a transverse plane within the body with approximately constant angular frequency, the phased array being positioned (1) to selectively focus the magnetic field B 1  within a selected interrogation volume of size no larger than 3,000 cm 3  within the body and (2) to selectively and preferentially receive response signals from the selected interrogation volume that are issued in response to magnetic resonance excitation of selected nuclei in the selected interrogation volume;   a first coil or other magnetic field structure for producing a primary magnetic field B 0  of approximately constant amplitude whose field vector is perpendicular to the transverse plane;   a second coil or other structure for producing a gradient magnetic field B 2 , parallel to the primary magnetic field whose amplitude is no larger than the amplitude of the primary magnetic field, where the gradient field distinguishes between changes of position within the body in at least two mutually orthogonal directions;   signal processing means for accepting the response signals received at each of the antennae as a function of time and for introducing a predetermined phase shift into each such response signal relative to the phases of response signals received at each of the other antennae;   a first switched power supply, connected to the second coil or magnetic field structure, to selectively activate and deactivate the gradient magnetic field at predetermined times; and   a second switched power supply to selectively activate and deactivate the signal processing means at predetermined times.   
     
     
       14. The apparatus of claim 13, further comprising an impedance-matching material positioned to fill substantially all the volume between said phased array of antennae and said body and to approximately match the electrical impedance of said body. 
     
     
       15. The apparatus of claim 13, wherein said primary magnetic field B 0  has a magnitude that lies in the range 2-10 Tesla. 
     
     
       16. The apparatus of claim 13, wherein said signal processing means comprises: first and second amplifiers, positioned to receive first and second response signals, respectively, that are received at a first antenna and a second antenna, respectively, in said phased array of said antennae, and to issue as output signals amplified first response signals and amplified second response signals, respectively;   first and second local oscillators for producing and issuing first and second local oscillator signals of predetermined frequencies;   first and second heterodyne receivers for receiving the first and second amplified response signals from the first and second amplifiers, respectively, for receiving the first and second local oscillator signals, for forming and issuing a first product signal that is a product of the amplified first response signal and the first local oscillator signal, and for forming and issuing a second product signal that is a product of the amplified second response signal and the second local oscillator signal;   first and second phase shift means for receiving the first product signal, for introducing predetermined first and second phase shifts, respectively, in the first product signal, and for issuing these phase shifted signals as first and second phase shift means output signals;   third and fourth phase shift means for receiving the second product signal, for introducing predetermined third and fourth phase shifts, respectively, in the second product signal, and for issuing these phase shifted signals as third and fourth phase shift means output signals;   a first signal summing device, positioned to receive and form the sum of the first and third phase shift means out put signals and to issue this sum signal as a first summing device output signal;   a second signal summing device, positioned to receive and form the sum of the second and fourth phase shift means output signals and to issue this sum signal as a second summing device output signal;   first and second analog-to-digital converters positioned to receive the first and second summing device output signals, respectively, to convert these signals to digital signals, and to issue these converted digital signals as first and second converter output signals   signal storage means for receiving and storing the first and second converter output signals;   transform means for forming and issuing a two-dimensional Fourier transform of the first converter output signal and for forming and issuing a two-dimensional Fourier transform signal of the second converter output signal; and   display means for receiving and graphically displaying the Fourier transform signals issued by the transform means.   
     
     
       17. The apparatus of claim 13, wherein said signal processing means comprises: first and second amplifiers, positioned to receive first and second response signals, respectively, that are received at a first antenna and a second antenna, respectively, in said phased array of said antennae, and to issue as output signals amplified first and second response signals;   first and second local oscillators for producing and issuing first and second local oscillator signals of predetermined frequencies;   first and second heterodyne receivers for receiving the first and second amplified response signals from the first and second amplifiers, respectively, for receiving the first and second local oscillator signals, for forming and issuing a first product signal that is a product of the amplified first response signal and the first local oscillator signal, and for forming and issuing a second product signal that is a product of the amplified second response signal and the second local oscillator signal;   first and second analog-to-digital converters, positioned to receive the first and second product signals, respectively, to convert these signals to digital signals, and to issue these converted signals as converted first and second product signals;   first and second memory means for receiving and storing the amplified and converted first and second product signals;   first and second phase shift means for receiving the converted first and second product signals from the first and second memory means, for introducing predetermined first and second phase shifts, respectively, in the converted first and second product signals, for issuing these phase shifted signals as first and second phase shift means output signals at a first selected time, for introducing third and fourth predetermined phase shifts, respectively, in the converted first and second product signals, and for issuing these phase shifted signals as third and fourth phase shift means output signals at a second selected time;   a signal summing device for receiving and forming the sum of the first and second phase shift means output signals at the first selected time, for issuing this sum as a first sum signal, for receiving and forming the sum of the third and fourth phase shift means output signals at the second selected time, and for issuing this sum as a second sum signal;   transform means for receiving the first and second sum signals, for forming and issuing a two-dimensional Fourier transform of the first sum signal as a first output signal, and for forming and issuing a two-dimensional Fourier transform of the second sum signal as a second output signal; and   display means for receiving and graphically displaying the first and second output signals issued by the transform means.   
     
     
       18. The apparatus of claim 13, further comprising steering means connected to said sensing antenna array for controllably causing said antenna array to illuminate a selected interrogation volume at a selected position within said tissue. 
     
     
       19. Apparatus for producing a time dependent magnetic field in a human or animal body for purposes of magnetic resonance imaging of tissue within an interrogation volume of reduced size no larger than 3,000 cm 3  in the body, and for sensing response signals issued by the tissue in response to receipt by the tissue of magnetic resonance excitation signals, the apparatus comprising: a phased array of antennae, having at least first and second antennae, positioned outside and adjacent to the body in a coronal plane, that produces a radiofrequency magnetic field B 1  with a field vector that rotates in a sagittal plane within the body with approximately constant angular frequency, the phased array being positioned (1) to selectively focus the magnetic field B 1  within a selected interrogation volume of size no larger than 3,000 cm 3  within the body and (2) to selectively and preferentially receive response signals from the selected interrogation volume that are issued in response to magnetic resonance excitation of selected nuclei in the selected interrogation volume;   a first coil or other magnetic field structure for producing a primary magnetic field B 0  of approximately constant amplitude whose field vector is perpendicular to the sagittal plane;   a second coil or other structure for producing a gradient magnetic field B 2 , parallel to the primary magnetic field whose amplitude is no larger than the amplitude of the magnetic field B 1 , where the gradient field distinguishes between changes of position within the body in at least two mutually orthogonal directions;   signal processing means for accepting the response signals received at each of the antennae as a function of time and for introducing a predetermined phase shift into each such response signal relative to the phases of response signals received at each of the other antennae;   a first switched power supply, connected to the second coil or magnetic field structure, to selectively activate and deactivate the gradient magnetic field at predetermined times; and   a second switched power supply to selectively activate and deactivate the signal processing means at predetermined times.   
     
     
       20. The apparatus of claim 19, further comprising an impedance-matching material positioned to fill substantially all the volume between each of said antennae of said first and second phased arrays of antennae and said body and to approximately match the electrical impedance of said body. 
     
     
       21. The apparatus of claim 19, wherein said primary magnetic field B 0  has a magnitude that lies in the range 2-10 Tesla. 
     
     
       22. The apparatus of claim 19, wherein said signal processing means comprises: first and second amplifiers, positioned to receive first and second response signals, respectively, that are received at a first antenna and a second antenna, respectively, in said phased array of said antennae, and to issue as output signals amplified first response signals and amplified second response signals, respectively;   first and second local oscillators for producing and issuing first and second local oscillator signals of predetermined frequencies;   first and second heterodyne receivers for receiving the first and second amplified response signals from the first and second amplifiers, respectively, for receiving the first and second local oscillator signals, for forming and issuing a first product signal that is a product of the amplified first response signal and the first local oscillator signal, and for forming and issuing a second product signal that is a product of the amplified second response signal and the second local oscillator signal;   first and second phase shift means for receiving the product first response signal, for introducing predetermined first and second phase shifts, respectively, in the first product signal, and for issuing these phase shifted signals as first and second phase shift means output signals;   third and fourth phase shift means for receiving the second product signal, for introducing predetermined third and fourth phase shifts, respectively, in the second product signal, and for issuing these phase shifted signals as third and fourth phase shift means output signals;   a first signal summing device, positioned to receive and form the sum of the first and third phase shift means output signals and to issue this sum signal as a first summing device output signal;   a second signal summing device, positioned to receive and form the sum of the second and fourth phase shift means output signals and to issue this sum signal as a second summing device output signal;   first and second analog-to-digital converters positioned to receive the first and second summing device output signals, respectively, to convert these signals to digital signals, and to issue these converted digital signals as first and second converter output signals;   signal storage means for receiving and storing the first and second converter output signals;   transform means for forming and issuing a two-dimensional Fourier transform of the first converter output signal and for forming and issuing a two-dimensional Fourier transform signal of the second converter output signal; and   display means for receiving and graphically displaying the Fourier transform signals issued by the transform means.   
     
     
       23. The apparatus of claim 19, wherein said signal processing means comprises: first and second amplifiers, positioned to receive first and second response signals, respectively, that are received at a first antenna and a second antenna, respectively, in said phased array of said antennae, and to issue as output signals amplified first and second response signals;   first and second local oscillators for producing and issuing first and second local oscillator signals of predetermined frequencies;   first and second heterodyne receivers for receiving the first and second amplified response signals from the first and second amplifiers, respectively, for receiving the first and second local oscillator signals, for forming and issuing a first product signal that is a product of the amplified first response signal and the first local oscillator signal, and for forming and issuing a second product signal that is a product of the amplified second response signal and the second local oscillator signal;   first and second analog-to-digital converters, positioned to receive the first and second product signals, respectively, to convert these signals to digital signals, and to issue these converted signals as amplified and converted first and second product signals;   first and second memory means for receiving and storing the converted first and second product signals;   first and second phase shift means for receiving the amplified and converted first product signals from the first and second memory means, for introducing predetermined first and second phase shifts, respectively, in the converted first and second product signals, for issuing these phase shifted signals as first and second phase shift means output signals at a first selected time, for introducing third and fourth predetermined phase shifts, respectively, in the converted first and second product signals, and for issuing these phase shifted signals as third and fourth phase shift means output signals at a second selected time;   a signal summing device for receiving and forming the sum of the first and second phase shift means output signals at the first selected time, for issuing this sum as a first sum signal, for receiving and forming the sum of the third and fourth phase shift means output signals at the second selected time, and for issuing this sum as a second sum signal;   transform means for receiving the first and second sum signals, for forming and issuing a two-dimensional Fourier transform of the first sum signal as a first output signal, and for forming and issuing a two-dimensional Fourier transform of the second sum signal as a second output signal; and   display means for receiving and graphically displaying the first and second output signals issued by the transform means.   
     
     
       24. The apparatus of claim 19, further comprising steering means connected to said sensing antenna array for controllably causing said antenna array to illuminate a selected interrogation volume at a selected position within said tissue.